1. Field of the Invention
The present invention generally relates to soldering leads, and more particularly, relates to laser soldering microelectronic leads.
2. Related Art
Mounting microelectronic devices on printed circuit boards has become a popular method for packaging circuits for units such as computers and control systems. Known methods for mounting microelectronic devices on printed circuit boards include surface mount and through-hole mount methods. Particularly, in surface mount methods, a microelectronic device is disposed on a first surface of a printed circuit board, and leads of the device are soldered to and form connections with pads deposited on the first surface of the circuit board. The connections are made on the same surface, i.e., the first surface, on which the microelectronic device is disposed. With through-hole mount methods, a microelectronic device is disposed on a first surface of a printed circuit board and leads of the device are aligned with and passed through openings formed in the circuit board. Then, the leads of the device are soldered in the opening and form connections with conductors disposed on the printed circuit board.
With surface mount devices, the number of leads extending from a microelectronic chip may exceed three hundred, for example. Lining the leads up with corresponding pads on the printed circuit board, and then maintaining the alignment during a soldering process is a difficult, and time consuming, task.
To facilitate aligning and maintaining the leads in position relative to pads on the printed circuit board, a process generally known as Tape-Automated Bonding (TAB) has been implemented. A microelectronic chip, its corresponding leads, and polyimide tape typically compose a TAB device. The leads are bonded to the tape, and then the chip is bonded to an inner portion of the each lead. Then, an outer portion of the tape is etched away so that the leads extend from an outer surface of the tape. The leads from the TAB device are supported, at least partially, by the tape to facilitate aligning the leads with solder plated pads. The leads are then soldered to corresponding printed circuit board pads.
Particularly, at the location where the tape has been etched away, the leads of the chip are exposed. These exposed leads are aligned with solder plated pads formed on the corresponding printed circuit board conductors, and the leads are then soldered using, for example, a hot blade. The hot blade is used for conductive heating to reflow solder plates on the solder pads. The hot blade is also utilized to hold the leads in contact with their mating pads while the solder is reflowed.
Lead hold down is a common problem in most solder processes, and the problem is amplified when soldering flexible leads, which are typical in TAB devices. In known TAB processes, the exact point where the soldering is to occur is the exact point where the leads no longer are supported or held by the tape as a result of the tape etching process. With known TAB approaches, if the tape were not removed, utilizing a hot blade would result in burning the tape and possibly damaging the chip due to excessive heat.
Other than using a hot blade, it is known to use a laser beam in a TAB process to reflow the solder. Particularly, using known laser-TAB methods, an outer portion of the tape is removed via an etching process, the leads are aligned with corresponding solder pads, and a laser beam is utilized to reflow the solder. Known laser tools which could be utilized in a laser-TAB process are described in commonly assigned U.S. Pat. No. 4,564,736, entitled "Industrial Hand Held Laser Tool and Laser System", commonly assigned U.S. Pat. No. 4,681,396, entitled "High Power Laser Energy Delivery System", and commonly assigned U.S. Pat. No. 4,799,755, entitled "Laser Materials Processing with a Lensless Fiber Optic Output Coupler".
In known laser-TAB processes, however, the problem of lead hold down is prevalent because the leads are unsupported by the tape at the location of soldering. At least with the hot blade method, the leads can be held down with the blade. Further, fumes result when utilizing a laser beam to reflow the solder, and the fumes can coat the laser beam delivery optics thereby deteriorating operation of the laser device.